This invention pertains generally to the field of electro-discharge machining and to micro-electromechanical devices and processes for producing such devices.
Micro-electro-discharge machining (micro-EDM) is a microfabrication technique that is well suited to cutting electrically conductive materials such as steel, graphite, silicon and magnetic materials. See, e.g., D. Reynaerts, et al., xe2x80x9cIntegrating Electro-Discharge Machining and Photolithography: Work in Progress,xe2x80x9d J. of Micromechanics and Microengineering, Vol. 10, No. 2, June, 2000, pp. 189-195; Y. Honma, et al., xe2x80x9cMicro-Machining of Magnetic Metal Film Using Electro-Discharge Technique,xe2x80x9d Advances in Information Storage Systems, Vol. 10, 1999, pp. 383-399; C. A. Grimes, et al., xe2x80x9cMagnetoelastic Microsensors for Environmental Monitoring,xe2x80x9d Tech. Dig., IEEE Intl. Conf. on Micro Electro Mechanical Systems (MEMS ""01), Interlaken, Switzerland, January, 2001, pp. 278-281. Micro-EDM involves the sequential discharge of electrical pulses between a microscopic electrode and the workpiece while both are immersed in a dielectric oil. See, generally, T. Masaki, et al., xe2x80x9cMicro Electro-Discharge Machining and its Applications,xe2x80x9d Proc., IEEE Intl. Conf. on Micro Electro Mechanical Systems (MEMS ""90), Napa Valley, Calif., February, 1990, pp. 21-26. The pulse discharge timing is controlled by a simple resistor-capacitor (RC) circuit. In conventional micro-EDM, the electrode is a cylindrical metal element from 5 to 300 xcexcm in diameter. Although micro-EDM has been used commercially for applications such as ink-jet nozzle fabrication, the traditional process is limited in throughput because it is a serial process. The use of a single electrode limits not only the throughput, but also precision, because the electrodes themselves are individually shaped by using a micro-EDM techniquexe2x80x94wire electrode-discharge grinding (WEDG)xe2x80x94and variation may occur in the electrode shape. See, e.g., T. Masuzawa, et al., xe2x80x9cWire Electro-Discharge Grinding for Micro-Machining,xe2x80x9d Ann. CIRP, Vol. 34, 1985, pp. 431-434.
To address the throughput and material issues that limit conventional micro-EDM, batch mode micro-EDM has been developed using LIGA-fabricated electrodes. The LIGA process uses x-ray lithography to form high aspect ratio molds for electroplated structures. For a general discussion of the LIGA process, see W. Ehrfeld, et al., xe2x80x9cLIGA Process: Sensor Construction Techniques via X-Ray Lithography,xe2x80x9d Tech. Dig., IEEE Intl. Conf. on Solid-State Sensors and Actuators Workshop (Hilton Head ""88), June, 1988, pp. 1-4. Electroplated copper electrodes formed using the LIGA process have been shown to provide acceptable wear resistance. K. Takahata, et al., xe2x80x9cA Novel Micro Electro-Discharge Machining Method Using Electrodes Fabricated by the LIGA Process,xe2x80x9d Tech. Dig., IEEE Intl. Conf. on Micro Electro Mechanical Systems (MEMS ""99), Orlando, Fla., January, 1999, pp. 238-243. Parallel machining to provide perforations in stainless steel by using 3xc3x974 arrayed electrodes with 100 xcexcm diameter and 500 xcexcm pitch was also demonstrated. Sequential application with electrode arrays has also been utilized to produce a 1-mm long WC-Co super-hard alloy mechanical processing tool. K. Takahata, et al., xe2x80x9cHigh-Aspect-Ratio WC-Co Microstructure Produced by the Combination of LIGA and Micro-EDM,xe2x80x9d Microsystem Technologies, Vol. 6, No. 5, August, 2000, pp. 175-178.
A micro-electro-discharge machining apparatus in accordance with the invention includes a substrate having an array of electrodes formed thereon, with a plurality of electrical interconnect lines formed on the substrate extending to each of the electrodes. One interconnect line may extend to a set of electrodes or, preferably, each interconnect line extends separately to a single electrode. An electrical power source has a terminal connected through a resistor to each of the interconnect lines and has its other terminal connected to the workpiece to be machined. A capacitor is connected between each of the interconnect lines and a terminal of the power source and forms, with the resistor, an RC circuit which charges each of the electrodes to a voltage level sufficient to provide an electrical discharge between the electrode and the workpiece. By utilizing multiple interconnect lines rather than a single conductive baseline to which all of the electrodes are connected, several electrodes can be charged individually to a voltage level sufficient to provide a discharge, greatly increasing the rate of machining and reducing processing time. Most preferably, each electrode is connected individually to an interconnect line which is connected through a resistor to the power source.
A discrete capacitor may be utilized as the charging capacitor, with one such capacitor connected between each of the interconnect lines and the positive terminal of the power source. The apparatus of the invention may also utilize a conductive substrate, such as doped silicon, on which an insulating layer (e.g., SiO2) is formed, with the interconnect lines and the electrodes formed on the insulating layer. A distributed capacitor is formed between each of the interconnect lines and the conductive substrate, and this capacitor may be utilized as the capacitor which is charged, eliminating the need for a separate discrete capacitor and allowing the discharge pulse current amplitude and duration to be controlled to a desired level to control the machining characteristics of the apparatus.
In accordance with the invention, distributed capacitors may be formed on a dielectric substrate by providing a conductive layer on the surface of the substrate, which acts as one plate of the distributed capacitors. An insulating layer is then formed over the conductive layer and the interconnect lines are formed on the insulating layer and form the other plates of the distributed capacitors. The resistors may also be integrated with the interconnect lines on the substrate, for example, by depositing polysilicon on the substrate which bridges the gap in and is thus connected in the interconnect lines, or by doping a segment of a semiconductor substrate to form a resistor which is connected in an interconnect line. By utilizing resistors formed on the substrate in each interconnect line, the number of contact pads required may be less than the number of interconnect lines, with one pad connected to several interconnect lines.
Further objects, features and advantages of the invention will be apparent from the following detailed description when taken in conjunction with the accompanying drawings.